A fundamental principle that governs good practice of monitoring and checking the food chain requires to perform check analyses as far upstream as possible from production in order to be able to identify and isolate as quickly as possible the foodstuffs suspected of being contaminated.
As a general rule, when detecting tests, often called “screening” tests, are performed, a sample that was detected positive during a first check analysis is in fact only assumed to be actually positive and it will have to be subjected to a second test called “confirmation” test. On the other hand, if an initial screening test gives a negative result, this is sufficient and no further analysis needs to confirm the result(1) again.
One first consequence of this rule is that the first screening method must be able to handle the detection of a maximum number of compounds. The detecting or screening tests should thus preferably and logically be “multi-analyte tests.”
A second consequence of this rule is that it is important to know the class to which belongs the compound discovered in a positive sample during the screening test so as to be able to directly switch to the confirmation method which is normally very specific since it is recommended that the compound concerned be isolated and identified.
A third consequence of this rule is that a screening test cannot give results of a “false negative” type since these will then elude analysis at this preliminary stage and will not be confirmed later.
Various methods for the detection of antibiotics are currently known:                microbiological tests that measure the inhibitive power of a sample on the growth of a bacterial strain. This type of test requires a relatively long incubation period (between 3 and 16 hours) before the result is obtained. In general, these tests (Delvotest® SP, BRT Test, Copan™, Eclipse™, Valio™) can simultaneously recognize several classes of antibiotics since the bacterial strains used are often sensitive to several compounds of different classes. However, this type of test does not allow to identify the precise class to which the antibiotic compound tested belongs;        for the detection of small molecules, in vitro tests work according to the principle of the competition between the compound sought that is present in the sample and a marked competitor that was deliberately introduced into the sample for a single recognition site that may either be a receptor or an antibody. In a formulation of the type ELISA (Enzyme Linked Immuno Sorbent Assay) or RIA (Radio Immuno Assay)/RRA (Radio Receptor Assay), the time required for an analysis is of the order of 2 to 6 hours. Some of these methods, in particular the RRA laboratory methods, allow the simultaneous detection of several compounds of different classes. In this case however, the method does not allow to identify the class to which the compound that gave the positive result belongs;        the more complex physico-chemical methods that allow to isolate and identify the compound sought have until recently been mainly methods where a system of chromatographic separation is combined with a mass spectrometry detection system (GC/MS or LC/MS). These methods require to adapt particular procedures for each distinct compound to be identified. Sometimes a single compound or some of the compounds of a same class may be simultaneously analysed, sometimes all the compounds of a same class may be detected but this is never possible with compounds of different classes. In fact, the principle of chromatographic separation is characteristic of the physico-chemical properties of a given compound, these properties often being different from one class to another. In the case where the operator does not know the type of compound to be identified, he must consider as many methods as there are classes of compounds.        
In recent years, much quicker methods have been developed. These methods, called “RAPID TESTS”, are used to perform rapid screening tests on a large number of samples. In general, these methods use the recognition of the compounds sought in relation to a biological molecule according to the competition principle. To make the analysis quick and easy, these types of test work with membranous devices with lateral flow (Tetrasensor™, SNAP®, Beta-STAR™, ROSA). These tests are classified according to various classes of antibiotics but none of the products known to date allow to detect compounds belonging to different classes in a single operation.
Even if it is reasonable to envisage and to impose on the agri-food sector primary screening analyses, the sector involved will demand the most complete analysis possible that would preferably identify a maximum number of suspect compounds. It is indeed more practical and economical to perform a single multiple test from a single sample rather than to have to perform a specific test for each specific compound. This practice requires a great deal of time, sample management and cost. It acts as a brake upon good management and effective control of foodstuffs.
At this stage, checking effectiveness is thus greatly restricted by the lack of multi-analyte tests that would allow to detect all of the compounds of several classes of analytes in a single operation and in less than 10 minutes, for example.
In particular, the agri-food industry would be interested in a new method allowing to analyse compounds belonging to at least two different classes of antibiotics in a single operation.
The type of antibiotic that may be administered to animals may vary depending on whether the application is therapeutic or prophylactic, on the animal species, on the germ to fight against, on the veterinary practice, on the legislation in force, on the available means or even on the geographical region. In the case of some specific treatments, a mixture of medication is used. As a general rule, the practitioner uses antibiotic products selected from among all the commercially available compounds as he assesses which are most effective.
The main classes of antibacterial agents and antibiotics used are: penicillins and cephalosporines, tetracyclines, sulfamides, aminoglycosides and aminocyclitols, macrolides, chloramphenicols or other peptides, ionophores, nitrofuranes, quinolones, carbadox, etc. All these classes cover a very wide range of chemically different compounds.
It is thought that the sometimes intensive use of antibiotics in veterinary medicine and in agricultural production may have caused the emergence of bacterial strains that have become resistant to antibiotics. In order to safeguard human health and to legislate in this field, many countries (European Union, USA, Canada, etc.) have set maximum authorized limits for the residues (MRLs—maximum residue level) of antibiotics in foodstuffs(2). To some extent these MRLs set the boundary between a positive sample and a negative one, i.e. between a rejected sample and an accepted sample.
In parallel, the Commission Decision of Aug. 12, 2002 established the minimum required performance limits (MRPLs) applicable to analytical methods to be used for examining samples and defined common criteria for the interpretation of results(3).
It is understood that the only analysis techniques for which it can be demonstrated, on the basis of verifiable proofs, that they are validated and have an error rate conforming to standard of less than 5% for the level considered, may be used for the purposes of screening in compliance with Directive 96/23/EC.
In 1995, an average of one percent of all samples analysed for their antibiotic content had a level higher than the MRL and gave a positive result. When these positive results were confirmed, the products most frequently involved were penicillins and tetracyclines(4).